Redox flow batteries store electrical energy in a chemical form and subsequently dispense the stored energy in an electrical form via a spontaneous reverse redox reaction. Conversion between the chemical and electrical energy occurs in a reactor cell. One issue with conventional redox flow batteries is that over time the electrolyte state of charge can become imbalanced, thereby decreasing battery capacity due to hydrogen generation from the electrolyte via side reactions. For example, hydrogen gas is emitted as an electrochemical byproduct during battery charging. As another example, in a hybrid redox flow battery, hydrogen gas is emitted as a byproduct of a corrosion reaction at a negative (plating) electrode. Because hydrogen gas production consumes protons instead of the electro-active material in the battery, hydrogen gas generation not only results in an electrolyte state of charge imbalance which reduces the battery capacity, but also a rise in electrolyte pH which can lead to electrolyte stability issues.
Electrolyte rebalancing methods and systems typically employ an auxiliary rebalancing cell (electrochemical or photochemical) to convert the hydrogen gas back to protons via an auxiliary electrochemical reaction. For example, Thaller (U.S. Pat. No. 4,159,366) discloses a redox flow system including an electrochemical rebalancing cell, where hydrogen gas evolved from the battery negative electrode flows through the rebalancing cell anode and positive electrolyte flows through the rebalancing cell cathode. Electrochemical reactions occurring at the electrodes of the rebalancing cell convert gaseous hydrogen back to protons, consume the imbalanced positive electrolyte, and rebalance the electrochemical capacity of the positive and negative electrolytes.
The inventors have recognized various issues with the above system. Namely, electrochemical fuel cells are complex systems that are costly to manufacture and to operate. A simpler and cheaper effective alternative to providing an auxiliary cell for rebalancing electrolytes in redox flow batteries is needed.
One approach that addresses the above issues is a method of rebalancing electrolytes in a redox flow battery system, comprising directing hydrogen gas generated on the negative electrode of the redox flow battery system to a catalyst surface, and fluidly contacting the hydrogen gas with the positive electrolyte comprising a metal ion at the catalyst surface. Such operation may occur with the positive metal ion chemically reduced by the hydrogen gas at the catalyst surface and/or a state of charge of the electrolyte substantially balanced. In one embodiment, the metal ion may comprise an imbalanced positive metal ion. In another embodiment, the electrolyte pH and the electrolyte state of charge remain substantially balanced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.